A. Hematopoiesis
The mammalian hematopoietic system represents the source for the continuous production of large numbers of mature cell populations, that collectively represent the different array of peripheral blood lineages. Transplantation studies over the last four decades have defined operationally the activities of a rare bone marrow stem cell that is multipotential in its ability to give rise to mature blood cells and has self-renewal potential. Thus, it is currently believed that hematopoiesis is sustained by uncommitted stem cells that generate committed precursors that are capable of producing mature blood cells of all lineages (Dieterlen-Lievre et al., Int Arch Allergy Immunol, 112, 3-8, 1997; Morrison et al., Development, 124:1929-39, 1997).
Hematopoietic stem cells are self-regenerating, and also pluripotent in that they differentiate into several lineages, including lymphoid, myeloid and erythroid lineages. The lymphoid lineage, comprising B-cells and T-cells, provides for the production of antibodies, regulation of the cellular immune system, detection of foreign agents in the blood, detection of cells foreign to the host, and the like. The myeloid lineage, which includes monocytes, granulocytes, megakaryocytes as well as other cells, monitors for the presence of foreign bodies in the blood stream, provides protection against neoplastic cells, scavenges foreign materials in the blood stream, produces platelets, and the like. The erythroid lineage provides the red blood cells, which act as oxygen carriers. Exposure to growth factors is believed to induce a stem cell to be dedicated to differentiate into a specific lineage.
Many strategies have been developed to physically purify hematopoietic stem cells from the bone marrow or other sources such as the peripheral blood or fetal liver (Goodell et al., J Exp Med, 183:1797-806, 1996; Goodell et al., Nat Med, 3:1337-45, 1997; Lemischka et al. Cell, 45:917-27, 1986; Spangrude et al., Science, 241:58-62, 1988; Wolf et al., Exp Hematol, 21:614-22, 1993). However, the identity of the stem cells has remained elusive, and the only reliable measure of stem cell activity in any type of purified stem cell population is by in vivo transplantation. Remarkably, in some cases, it has been possible to demonstrate that a single stem cell is both necessary and sufficient to reconstitute a normal hematopoietic system into a recipient host (Jordan and Lemischka, Genes Dev, 4:220-321990; Osawa et al., Science, 273:242-5, 1996).
Recently, several studies have provided evidence for the existence of stem cells in several somatic tissues (Doetsch et al., Cell, 97:703-16, 1999; Gussoni et al., Nature, 401:390-4, 1999; Jackson et al., PNAS USA, 96:14482-6, 1999; Johansson et al., Cell, 96:25-34, 1999; Potten et al., Int J Exp Pathol, 78:219-43, 1997; Reynolds and Weiss, Science, 255:1707-10, 1992). Somatic tissues were previously believed to contain somatic stem cell populations capable of self-renewing potential but with a limited degree of lineage plasticity. However, transplantable hematopoietic cells were recently identified from muscle (Gussoni et al., Nature, 401:390-4, 1999; Jackson et al., PNAS USA, 96:14482-6, 1999).
Hematopoiesis is governed by a complex set of cytokines that variously regulate stem cell functions (Dieterlen-Lievre, Curr Biol, 8:R727-30, 1998; Ogawa, Blood, 81:2844-53, 1993). Individual cytokines and various combinations of these cytokines have been shown to maintain and promote the differentiation of populations of stem cells. In the mouse, cytokines have also been shown to modesty expand stem cells that are capable of long-term hematopoietic reconstitution of recipient animals (Dieterlen-Lievre, Curr Biol, 8:R727-30, 1998; Ogawa, Blood, 81:2844-53, 1993). However, the inability to consistently sustain and expand hematopoietic stem cells in vitro and in vivo represents a major barrier for further understanding the biology of stem cells and the ability to utilize such cells for therapeutic interventions. For example, use of chemotherapeutic agents for cancer therapy is often limited by hematopoietic stem cell toxicity and gene therapy approached to correction of genetic deficiencies is hampered by efficient gene transfer of cells with self-renewing potential (Dick, Nat Med, 6:624-6, 2000; Halene and Kohn, Hum Gene Ther, 11:1259-67, 2000; Mulligan, Science, 260:926-32, 1993). Thus, there is a need for identification of novel agents that promote stem cell survival and proliferation.
B. Calreticulin
Calreticulin was first identified in skeletal muscle sarcoplasmic reticulum. (Ostwald and MacLennan, J. Biol. Chem. 249 (3):974-979, 1974). Fifteen years later it was cloned and the N-terminus was sequenced. This led to the discovery that several groups had independently identified the molecule and had given it different names, including, “high-affinity Ca2+”, “calregulin”, “CRP55” and “calsequestrin-like protein” (Ostwald and MacLennan, J. Biol. Chem. 249 (3):974-979, 1974; Waisman et al., J. Biol. Chem. 260(3):1652-1660, 1985; Macer, D. R. J. & Koch, G. L. E. J. Cell. Sci. 91:61-70, 1988; Damiani et al., Biochem Biophys Res Commun 165(3):973-980, 1989; Treves et al., Biochem. J. 271:473-480, 1990). Each of these groups identified calreticulin through different means, but all identified its ability to bind Ca2+.
Although most studies have indicated that calreticulin resides predominantly within the lumen of the endoplasmic reticulum, calreticulin may also be found in other cellular compartments. For example, calreticulin was detected on the plasma membranes of lymphoblastoid cells (Newkirk and Tsoukas, J. Autoimmun. 5:511-525, 1992) and epidermal keratinocyte lines (Kawashima, et al., Dermatology 189 Suppl. 1:6-10, 1994). It was proposed to represent, or to be closely related in structure, to the C1q receptor found on endothelial cells, B cells, T cells and other cells (Chen et al., J. Immunol. 153:1430-1440, 1994). Calreticulin is also a constituent of lytic granules contained in cytotoxic T and NK cells from which it is released during cell lysis (Dupuis et al., J. Exp. Med. 177:1-7, 1993), and has been purified from the culture supernatant of several cell types (Booth and Koch, Cell 59:729-737, 1989; Eggleton et al., Clin. Immunol. Immunopathol. 72:405-409, 1994) and from normal human plasma (Sueyoshi et al., Thromb. Res. 63:569-575, 1991). Several observations support the notion that calreticulin can also be a target for autoimmune responses (Lux et al., J. Clin. Invest. 89:1945-1951, 1992; Meilof et al., J. Immunol. 151:5800-5809, 1993).
Since the initial identification and cloning, the structure of calreticulin has been characterized. Mammalian calreticulin is a 417 amino acid peptide from which the 17 N-terminal amino acids are cleaved upon translocation to the lumen of the endoplasmic reticulum (Smith and Koch, Embo. J. 8(12):3581-3586, 1989). In addition to being found in the lumen of the endoplasmic reticulum, calreticulin has been found in the cytoplasm, in the nucleus of some cells, and in the extracellular matrix (Michalak et al., Biochem. J. 285:681-692, 1992). Further studies revealed that calreticulin has three distinct domains, the N-terminal domain, a middle domain and the C-terminal domain.
The mature calreticulin is composed of an N-terminal domain consisting of 180 amino acids that are highly conserved. Proposed three-dimensional models indicate that the domain contains eight anti-parallel β-strands. Furthermore, the N-terminal domain has been found to bind a number of molecules including the alpha subunit of integrin, Zn2+, and the DNA binding domain of steroid receptors (Nash et al., Mol. Cellular Biochem. 135:71-78, 1994). The N-terminal domain of calreticulin is also know as vasostatin (vaso) (Pike et al., J. Exp. Med., 188:2349-56, 1998).
The middle domain of calreticulin stretches from amino acid 181 to amino acid 280. It is proline rich and has also been termed the P-domain. This domain has been found to have a high affinity for Ca2+ and contains a nuclear localization signal (Baksh and Michalak, J. Biol. Chem. 266:21458-21465, 1991).
Following the P-domain is the C-domain that includes the terminal 290-417 amino acids. This last domain is highly acidic and contains an endoplasmic reticulum retention signal. The C-domain binds to Factor IX, Factor X, and prothrombin (See U.S. Pat. No. 5,426,097, to Stern et al.).
Calreticulin has also been found to be useful in wound healing (See U.S. Pat. No. 5,591,716, to Siebert et al.).